This invention relates to a motor for applying a driving torque to a drive shaft.
More particularly, this invention relates to an electric motor adapted for direct drive of a take-up reel in a tape recorder.
In an electric motor in which a rotating magnetic field produced by driving coils interacts with a rotor magnet to produce a torque, excessive load can cause such a large electric current in the driving coils that heating and possible coil burnout can occur. Furthermore, the torque which can be produced by such a motor can exceed a desired maximum torque.
These problems are especially severe in a direct drive reel motor used, for example, to drive a take-up reel in a cassette tape recorder. When the end of the tape is reached on the supply reel, the take-up reel, driven by the reel motor, is suddenly stopped and may remain in this condition for an extended period in some equipment. Thus, a direct drive reel motor connected to the take-up reel may experience extreme and long sustained overload. If the rotor of the reel motor is stalled by such overload, high values of current can heat and damage its drive coils. In addition, the available torque exerted on the take-up reel can apply such an excessive force to the tape at the hub of the supply reel that the tape may be broken.
A need exists therefore for a novel motor which is not damaged by overload and which does not develop excess torque.